Public transport?

I never figured out why so many of my economically conservative friends are opposed to investing in public transport. After all, we subsidise private transport in the form of roads, bridges, highways and public parking. Public transport takes load off this infrastructure, which means we need to invest less into it. This could make heavy investment into public transport revenue positive. Why upgrade a city road to half a dozen lanes to reduce congestion, when the allocation of a single bus lane is cheaper? Some question the compatibility of this view with my generally pro-market, Libertarian-leaning views. However, these views are completely consistent. Libertarians believe in reducing tax-payer burden and public transport is one of the few areas where tax-payer spending can reduce the overall tax burden.